So I had a two battery mod that failed out on me. When I bought batteries for that mod originally I used those batteries just for that mod even though I had a single battery mod. I also always charged them at the same time. Over the time that I have read so many posts on ECF, I came across a concept that if I do have a multiple battery mod my batteries have to be married. I don't believe I understand this concept well enough. When my two battery mod failed I did use those two batteries individually for my single battery mod. Does this mean that I need to buy two new batteries when I buy my next two battery mod? Please explain this to me because I don't understand the married batteries concept. Thank you for your time.